1. Field of the Invention
This invention relates to a magnetic holding device for holding and releasing a magnetic substance. More specifically, the inventive magnetic holding devices include a permanent magnet and an exciting coil.
2. Description of the Prior Art
A prior art magnetic holding device employing a permanent magnet and an exciting coil keeps a holding portion in an excited state through the action of the magnetic flux of the permanent magnet when holding the magnetic substance. The holding portion is kept in an unexcited state through the action of the exciting coil in generating a the magnetic field to prevent the magnetic flux of the permanent magnet from acting on the holding portion when the magnetic substance is to be released.
In this known magnetic holding device, the magnetic flux from the permanent magnet is allowed to pass through a magnetic cavity acting as a magnetic leakage path by a magnetic field generated by the exciting coil while the holding portion is maintained in an unexcited state, but this magnetic flux is prevented from passing through the holding portion.
As a result of the variance in the magnetic properties of materials used and the processing accuracy of parts, the holding portion of this prior art magnetic holding device cannot be kept in a completely unexcited state.